1. Field of the Invention
The present invention relates to measurement systems using high (fast) energy neutron sources, and direct measurement of transmitted fast neutrons which have transited through a measurand.
2. Description of the Prior Art
Fast or high energy neutron sources are known, and are used as transmitting sources in measurement systems, such as moisture detectors. Fast neutrons may be converted to thermal or slow neutrons when they travel in certain materials, especially hydrogen containing materials. Detection, of either backscattered or transmitted thermal neutrons is the basis of such measurements. Operating limits for wall and material thicknesses, sensitivity to system geometry, and capture of thermal neutrons by other elements (e.g. chlorine, boron) in the measured material limit the application and accuracy of such thermal neutron measurement systems. Further, the basis for such measurements are inferred from the quantity of transmitted thermal neutrons detected, based on a quantity of available fast neutrons from the source. Fast neutrons are less subject to scattering or capture by other materials and have greater probability of penetration through such materials than do thermal neutrons. Prior systems have not made use of the advantages offered by the direct measurement of the quantity of fast neutrons for process control and measurements. The use of thermal neutron backscatter, thermal neutron transmission and various gamma radiation techniques for process measurements have long been known. For example, the assignee of the present application, Kay-Ray, Inc. of Arlington Heights, Ill. has made level and interface measurement systems using radiation sources and detection, such as their Model 4800X Level System, the Model 4760 Level System, the Model 4160 Level/Interface Measurement System, and the Model 3660 Density Measurement System. These systems use radiation principles, having sources of radiation and detectors, which provide an output that can be sensed by detection circuitry and then used for control, such as use with microprocessors and two or four wire transmitter systems. These systems all provide noncontact measuring, but rely upon gamma or thermal neutron radiation detection which rate susceptible to absorption by thick walled vessels.
The present invention of measuring the quantity of fast neutrons which have transited through the measurand has adaptation and uses in difficult measurement environments, such as relatively large distances, thick vessel walls, material build-ups on the interior of vessel walls, or thick layers of insulation which greatly affect other types of measurement systems. Fast neutrons do not occur naturally as background radiation, thus it is practical to provide and sense low quantities of fast neutrons. Hence radiation safety considerations are enhanced and potential hazards are minimized and its still possible to provide an adequate quantity of fast neutrons for desired measurement results.
In the prior art, British Patent No. 1,387,007 discloses a fast neutron source made of AmBe(Americum-241) Beryllium, but utilizes thermal neutron detection techniques to circumvent the layer thickness or product profile variations that would be encountered if backscatter measurement techniques were used. Density correction is also incorporated into the measurement by use of gamma absorption in this device. The patent also discloses the use of hydrogen reflectors to increase detector output. This system teaches the transmission of high energy neutrons through a conveyor belt and sample material, and as moisture varies in the product the quanitity of high energy neutrons converted to thermal neutrons varies, and the resulting thermal neutron flux variations are measured and related to moisture content. This configuration limits the amount of material that can be measured since when the material thickness is increased beyond certain limits, beyond such limit, the mean travel distance of the thermal neutrons through the material is exceeded. Hence, the thermal neutron flux is then no longer representative of the measurand.
British Patent No. 1,180,450 shows neutron backscatter techniques for measuring moisture, and deals with increasing the capability of making such measurements accurately through the use of hydrogen reflectors.
Other art shows various detection techniques used for neutron radiation, for example U.S. Pat. No. 3,532,883 teaches moisture detection of solids using neutron radiation and measurement of thermal neutrons transmitted through the material.
An article by Lepper, et al entitled "Nuclear Methods for Determining the Water Content and Unit Weight of Fresh Concrete", J. Materials--Volume 6, No. 4 (Dec. 1971), teaches a method of determining moisture level in solid and concrete material using a fast neutron source and thermal neutron counting device.
DEAK publications DL0158142 and DL0201514 teach methods of measuring moisture in concrete walls by neutron emission. Publication S3225-0002, teaches a method to determine moisture in a deep bore hole by means of using a fast neutron source. However, none of these references teach the measurement of transmission of high energy or "fast" neutrons from a source on one side of a vessel, and directly receiving the fast neutrons or high energy flux on the other side of the vessel and measuring this high energy flux variation directly.
German Patent Application No. 3,028,963 shows a source which emits both fast neutrons and gamma rays through a product, with detectors on the opposite side. The arrangement is used for calculating moisture content of a product on the basis of both the neutron and gamma ray counts. The application does state that neutrons and gamma rays may emanate from separate radiation surces, but both types of radiation are necessary to carry out the intent of the invention. Thus, the detectors used have to be able to sense both neutrons and gamma rays for obtaining or providing the desired output. In the present device, care is taken to insure that the detector is responsive only to the received fast neutrons, which are then converted to thermal neutrons for sensing. The German patent application makes it clear that the gamma rays are required in order to overcome the influence of bulk density of the material in the moisture measurement system disclosed therein.